It is common practice to utilize thermal checking to identify specific failed components in electronic circuitry, and to ensure operation at specific, cold temperatures. A typical example of large complex circuitry may include a plurality of individual components and integrated circuits mounted on a "mother" board (typically a printed circuit board which physically and electrically integrates the components into a subsystem). When such circuitry is intended for military, aerospace or other critical end use, it is common to perform many tests thereon prior to assemblage into an overall system, including thermal checks to ensure operation at cold temperatures. Such circuitry may be interconnected with very complex test equipment, which performs series of tests, the results of which indicate probable causes for certain malfunctions, but which cannot isolate faults in all cases. In such cases, it has also been known to perform thermal checks on the circuitry while it is connected to test equipment, to see if the thermal checking will locate the fault. Although the phenomenon is not entirely understood, it is believed that one type of fault which is overcome through thermal checking is minor cracks in conductors which become reconnected when cooled to temperatures in the range of minus tens of degrees centigrade. In such cases, the temporary correction of the fault will provide an indication of proper operation within the test equipment, thus indicating a component or circuit area which is probably at fault. Further testing and/or replacement of components or portions of circuitry is then undertaken to cure the defect.
Heretofore, it has been common to use coolants which are generally referred to herein as chlorofluorocarbons (CFCs) of which there are a large number of varieties. A most common variety is dichlorodifluoromethane, which is also known as Freon 12 and Halon 22. The CFC is typically applied from an aerosol can (much like a hairspray can) which can be carried by workers in a tool pouch.
For some years, scientists have been concerned about the effects of CFCs on the atmosphere. First, CFC molecules themselves trap 20,000 times more heat than a molecule of carbon dioxide, thereby increasing the greenhouse effect far out of proportion to its concentration in air. More importantly, chlorine released when CFC molecules break up combines with and destroys ozone molecules. And each chlorine atom can eventually be re-released and combined with yet other ozone molecules so that their destructive effect is repetitive, perpetually. And, as is known, it is the ozone molecules which absorb most of the ultraviolet radiation from the sun, which is known to be extremely harmful to all forms of animal life, from humans down to the simplest of forms. For that reason, many governments of the world are now restricting, with the ultimate aim at totally banning, the production and use of CFCs.
Some attempts have been made to provide alternative methods of performing thermal checks on circuitry. Heat pumps have such minor cooling as to be unable to reach the desired temperatures (on the order of -30.degree. to -60.degree. C.) in even five or six minutes. Expansion of high pressure gases can produce temperatures as low as -30.degree. C., but the high pressure gas causes physical damage to the circuitry under test. Thus far, no reasonable substitute seems to be available.